Electrostatically focused traveling wave tubes



Dec. l0, 1963 E. F. BELoHouBEK ELECTROSTATICALLY FOCUSED TRAVELING WAVE TUBES 2 Sheets-Sheet 1 INVENTOR. Erwm E elohoubeK Dec. 10, 1963 E. F. BELoHoUBl-:K 3,114,072

ELECTROSTATICALLY FOCUSED TRAVELING WAVE TUBES Filed May 5l, 1960 2 Sheets-Sheet 2 Il A I 5!! i3 INVENToR. Enum F. Belohoubele Wm Uni United States Patent O 3,114,072 ELECTROSTA'HCALLY FQSUSED TRAVEMNG WAVE TUBES Erwin F. Belononlrelr, lranldin Parli, NJ., assigner to Radio Corporation or' America, a corporation of Delaware Filed May 3l, 1959, Ser. No. 33,653 il Claims. (Cl. SiS-3.5)

The present invention relates to electrostatically focused traveling wave tubes.

The trend in many present day traveling wave tube applications is towards light weight, low beam voltage and high power output. Periodic focusing of the electron beam provides means for greatly reducing the weight of the final tube assembly. Of the two basic light weight possibilities, periodic magnetic focusing and periodic electrostatic focusing, the latter is superior especially for tubes in .the lower wavelength range of the microwave spectrum, where the periodic magnetic assembly represents the major part of the total weight of the tube. Besides `an appreciable reduction in weight, electrostatically focused tubes also offer other distinct advantages. First, they are able to work over a very wide range of temperatures (the temperature compensation of magnets being restricted to a relatively narrow range). Also, electrostatically focused .tubes are not `subject to ion bombnrdment of the cathode.

Earlier work on periodic electrostatic focusing was largely confined to bi-filar helices. Helices are, however, severely limited in their power handling capabilities and, therefore, a strong need exists for high power slow wave structures which can accommodate electrostatically focused beams. Most periodic slow-wave structures, excluding structures of the helix type, comprise some kind of transverse metal loading elements or partitions, periodically spaced in the axial direction, close to .the electron beam. Such a structure can propagate waves therealong having predominantly axial electric field Icomponents. ln one type of slow wave structure, known as the disk-loaded waveguide, a hollow metal waveguide is provided with centrally-apertured radial disks or irises periodcally spaced along the interior thereof. This type of slow ywave structure has a fundamental space harmonic with a forward wave characteristic, and has been used un'th a central solid beam as a linear accelerator.

The principal object of the present invention is to incorporate periodic electrostatic focusing in a slow wave structure of the disk-loaded waveguide type.

Another object of the invention is to adapt such a structure to broad-band operation in a high power traveling wave amplifier tube.

The changes in the disk-loaded waveguide that are necessary to provide direct current (DC.) isolated focusing elements for periodic electrostatic focusing of the beam affect the radio frequency (RF) behavior of the slow wave structure. For best results, the overall structure should be made in such way that:

(l) The RF radiation caused by the D.C.isolation of adjacent `loading elements is kept low;

(2) The tube will operate over a broad band of signal frequencies -with relatively low beam velocity;

(3) Any new modes originating from the D.C.isola tion of the loading elements `are suppressed or damped sufficiently to avoid unwanted oscillations;

(4) The interaction impedance of the working pass band of the structure is not lowered excessively; and

(5) The loading elements are capable of handling the intercepted beam current.

Previous attempts to incorporate periodic electrostatic focusing in `disk-loaded waveguides have ignored the radiation problem, and failed to solve the wave propagation problems in the structure.

In accordance with one embodiment of the invention, a hollow electron beam is projected axially in the annular space between a disk-loaded round waveguide and a coaxial cen-ter conductor, the 'annular loading disks being D.C.isolated from each other but RF-short-cirouited at their peripheral regions by intermediate ceramic rings of high dielectric constant having a radial thickness equal to a quarter wavelength in Athe middle of the operating frequency range. Alternate disks are connected together in two sets which are maintained at 4two different positive DC. potentials for periodic electrostatic focusing of the beam. Alternatively, an annular array lof solid beams may -be projected through a similar array of aligned apertures in the loading disks near the central opening thereof.

In the accompanying drawing:

FiG. l is an axial sectional view of a conventional slow wave structure of the disk-loaded waveguide type;

FiG. 2 is an wdiagram, or wave propagating characteristic, of the structure of FIG. l;

FIG. 3 is an axial sectional View of a traveling wave tube incorporating la disk-loaded waveguide according to one embodiment of the present invention;

FIG. 4 is a transverse sectional view taken on the line 4 4 of FIG. 3;

FlG. 5 is an fdiagram of the tube of FIGS. 3 and 4;

FIGS. 6 and 7 are axial and transverse sectional views of an alternative embodiment of the invention; and

FIG. 8 is an fdiagram for the structure of FIGS. 6 and 7.

FiG. 1 shows a conventional disloloaded waveguide made up of a hollow -cylindrical metal waveguide l in which a series of equally-spaced centrally-apertured disks 3 is mounted. This structure will propagate a wave axially therealong at a velocity somewhat less than the velocity of light with predominantly axial electric fields between adjacent loading disks 3. For use in a traveling wave tube, an electron beam'S is projected from a cathode 7 along the longitudinal axis of the loaded waveguide at a velocity equal to or slightly greater than the velocity of the wave for traveling wave interaction therewith.

FlG. 2 shows the w-,B diagram of the disk-loaded waveguide of FIG. l, where w=21r], f is the wave frequency, is defined by o is the phase shift per unit length of the line for the `fundamental space harmonic, n is the number of the space harmonic involved (11:0 for the fundamental space harmonic) and L is the RF cell length. In FIG. l, the fundamental space harmonic -is shown as curve A, together with a bearn velocity line, vb, for operation as a forward wave amplifier in which the phase and group velocities are in the same direction. Any straight line through the origin in FlG. l is a constant phase velocity line, since v=w/, the velocity being measured by the slope of the line. The velocity of light is indicated by the steeper line vp=c.

Traveling wave interaction between the beam and a wave traveling along the disk-loaded waveguide occurs only at or near the frequency at which the vb line intersects curve A. The relative slopes of the line and curve are such that interaction is limited to a relatively narrow frequency band at a given beam velocity. Moreover, the beam velocity is too high for practical use as an amplifier. No beam focusing means is shown in FIG. l. Normally, the waveguide would be surrounded by a solenoid or periodic magnetic system for confining the beam to substantially parallel flow.

FIG. 3 shows a traveling wave tube embodying a diskloaded waveguide adapted for electrostatic focusing and also for wideband, low-voltage operation. The tube comprises an elongated cylindrical center conductor il coaxially surrounded by a series of equally-spaced centrally apcrtured metal loading disks 13. The disks i3 are mechanically connected together at their Outer peripheries by ceramic rings of rectangular cross-section hermetically sealed to the disks. In addition to forming part of the vacuum envelope and insulating adjacent loading disks 13 for direct currents, to permit lthe application of different D.C. potentials thereto for electrostatic focusing, the ceramic rings 15 are adapted to provide a low impedance electrical connection for RF currents between adjacent loading disks 13. For this purpose, the radial thickness of the ceramic rings 15 is made a quarter-wave length in the middle of the desired operating range of the tube. In order to effectively short-circuit adjacent loading `disks 13 for RF currents over a wide frequency band, the ceramic rings 15 are made of a ceramic having a high dielectric constant. Best results have been obtained with Alsimag 192 ceramic, which consists essentially of titanium dioxide and has a dielectric constant of about 85. However, useful results can be obtained with materials having a dielectric constant as low as 25.

The tube includes an annular gun 17 arranged to project a hollow cylindrical beam of electrons through the annular space between the center conductor `11 and the disks 13. The gun 17 comprises a ring cathode 19 having a heater 21, an annular beam forming electrode 23, and an annular accelerating electrode 25. As shown, the gun electrodes are mounted by means of insulating rings 27 hermetically sealed to the electrodes and to the flirst loading disk 13. Insulators 29 connect the central portions of the electrodes to each other and to the center electrode il. An end plate 31 completes the -vacuum envelope behind the heater 2.1. The annular beam is collected by a convergent collector 33 surrounding a tapered extension 3S of the center conductor ll and brazed to the last disk 13".

The tube may be coupled to an input signal source (not shown) by means of a coaxial line 37 terminating in a loop 39 ymagnetically coupled to the first RF cell of the disk-loaded wave-guide. The collector 33 and extension form a coaxial `line coupling to lwhich a rectangular output waveguide is coupled. One major `wall of the waveguide is brazed to the outer end of the collector 33. The other major wall is mechanically connected to the extension 35 by an insulating ring 43, and electrically connected thereto for RF currents by means of a waveguide flange and a flanged plate 47 on extension 33 forming a wide-band double choke made up of a quarter-wave open line opening linto a quarter-wave closed line.

In operation, the loading disks 13 are connected, in two sets of alternate disks, to a D.C. voltage source 49 to maintain the two sets of disks at two different positive DC. potentials, in order to focus the annular beam. The center conductor 11 is maintained by source 49 at a DC. potential approximately midway between the two disk potentials. Suitable potentials may be supplied to the gun electrodes by source 49 as shown, or by a dilerent source.

By way of example only, one disk-loaded waveguide structure built according to FIG. 3 had the following dimensions; disk diameter 2.715", disk aperture 1.060", cell length .209, spacing between disks and axial dirnension of ceramic rings .175", radial thickness of ceramic rings .1075", and diameter of center rod .800. FIG. 5 shows the wave propagation characteristics of this particular structure. The dotted curve B shows the variation of with frequency up to about 4500 megacycles for t ie loaded waveguide structure with a metal outer wall (as in FIG. l) and a center conductor. Comparison of curve B with curve A of FIG. 2 shows that the center conductor causes the ,vf-,S characteristic to start from the origin, which permits very wide-band interaction with a beam.

The solid curve C is the f-,B characteristic for the example given, with the `metal outer wall replaced by 1A wave ceramic rings as shown in FIG. 3. It can be seen that the introduction of the ceramic rings, to permit D.C. isolation of adjacent loading elements for focusing purposes, produced very little cha-.nge in the wave propagation characteristic of the RF structure, particularly in the frequency range between 2 and 3 kmc., which is best suited for wideband interaction with the beam. Moreover, the slope of the beam velocity lline vh in FIG. 5 is somewhat less than the corresponding line in FIG. 2, which means lower voltage operation. It will be understood that the particular beam velocity shown in FIG. 5 is merely illustrative, and that interaction may be had between still lower velocity beams and the circuit at higher frequencies, but over narrower bands. Because of the wide-band RF short produced by the /i wave ceramic rings, the RF radiation that would otherwise result from the use of insulators between the loading disks is kept very low. Moreover, tests have shown that the interaction impedance of the working pass band of the structure is not lowered more than 6% over a frequency range of i 20%. The RF structure of FiG. 3 retains the capacity of the basic disk-loaded waveguide structure of FIG. 1 to handle residual intercepted beam current.

Because it is strongly dispersive at low voltages, the RF structure shown in FIG. 3 requires fairly high beam voltages for very wide band interaction with the beam. By increasing the diameter of the center conductor 11 until it comes close to the inner diameter of t-he loading disks 13 the structure could be made less dispersive. However, in this case the annular gap for the hollow beam would be too small to pass the beam and the axial RF electric field would be reduced.

A modied form of the invention which permits operation at much lower beam voltages with little dispersion is shown in FG. 6. ln this tube, an elongated cylindrical center conductor 51 is closely surrounded by a series of equally-spaced, centrally-apertured, loading disks 53 insulated apart at their outer peripheries by 1i-'wave choke ceramic rin-gs S5 of high dielectric constant, as in FIG. 3. The disks 53 are formed near their central apertures with an annular array of axially-aligned openings 57 (8 in each disk, as shown). A cathode 59', bearmforming electrode 6.1, and accelerating electrode 63 are mounted in alignment with each of the openings 57 to project an electron beam through the RF structure, for traveling wave interaction with the signal fields therein. The lgun electrodes may be mounted by insulating rings 65 sealed to the electrodes and to the rst loading disk 53. 'Ilhe gun portion of the vacuum envelope is completed by a plate 67 sealed to the end ring 65 to a tubular outward extension 69 of the first disk 53. After passing through the openings 57, the beams are collected by a collector 71 sealed to the outer surface of the last disk 53" and to a tubular outward extension 73 thereof. The center conductor 5I is provided at its ends with axial extensions 75 and '77 of reduced diameter which cooperate with the concentric tubular extensions 69 and 73, respectively, to form coaxial line input and output couplings for the tube. The outer ends of the tubular extensions 69 and 73 may be closed vacuum-tight by low-loss ceramic rings 79 and Si.

The fcharacteristic of the structure shown in FIG. 6 depends, in part, on the ratio of d, the distance between the center conductor 51 and the disks 53, to D, the radial depth of each RF cell. FIG. 8 shows the characteristic curve E for the structure of FIG. 6 with the dimensions given above as an example for the structure of FIG. 3. Thus, curve E is substantially the same as curve C of FIG. 5. In this example, the ratio cl/D is 2/15. Curve F is the characteristic for a structure having the same dimensions except for the diameter of the disk apertures which is reduced to .960, in which case the ratio d/D is only l/16. Thus, by making d/D very small in PEG. 6 the propagation characteristic can be made nearly straight at a relatively low slope over the major portion of the pass band of the structure. This means efficient interaction between a low velocity beam, vb, and the slow wave structure over a very wide bandwidth. Moreover, the plural beam arrangement of FIG. 6 also increases the interaction impedance of the structure because the solid beams Iare surrounded by axial RF fields, as compared to the hollow beam of FIG. 3 which is exposed to mainly transverse RF elds on the side facing the center conductor.

Curves C, E and F in FIGS. 5 and 8 are only plotted down to about 2 kmc. Below that point the curves would begin to change direction because the ceramic rings i5 would no longer provide a good RE short between the disks 13.

The use of a hollow beam, or a series of solid beams, instead of a single solid beam as used heretofore in most traveling wave tubes with periodic slow wave structures,

increases the average beam power. Thus, the invention is particularly suitable for high average power or continuous-wave (OW.) applications. It is believed that tubes can be designed incorporating the invention capable of delivering average output power of the omer of 100 kw. at S band (2-4 kmo).

The present invention is primarily intended for use in an ampliiier, because of the wide-band feature described. However, it will be understood that a traveling wave tube embodying the invention may also be used as a forward wave oscillator by feeding back a portion of the output energy to the input of the tube, or as a backward wave oscillator by causing the beam to interact with a negative space harmonic instead of the fundamental space harmonic.

What is claimed is:

1. A traveling wave tube including: a slow wave structure comprising an elongated conductor, a series of identical equally-spaced loading plates having aligned central apertures surrounding said conductor in spaced relation thereto, `and low impedance means eectively connecting peripheral regions of adjacent loading plates for radio frequency currents while insulating said plates from each other for direct currents; and means for projecting electrons 'along said structure in paths near the boundaries of said apertures in said loading plates for interaction with a wave propagated along said structure.

2. A traveling wave tube as in `claim l, wherein said electron projecting means comprises means for producing and directing a hollow beam of electrons through said apertures in the space between said conductor and said loading plates.

3. A traveling wave tube as in claim 1, wherein said loading plates are formed with a series of sets of axiallyaligned beam openings closely surrounding said central apertures, and said electron projecting means comprises means for producing and directing a separate electron beam through each set of aligned beam openings.

4. A traveling wave tube including: a slow wave structure, comprising an elongated conductive rod, a series of identical equally-spaced loading disks having aligned central yapertures coaxially surrounding said rod in spaced relation thereto, and a series of identical ceramic rings of high dielectric constant each coaxially interposed between and joined to two adjacent `ones of said loading disks at an annular region spaced from said apertures and having a radial thickness substantially equal to a quarter Wavelength in the ceramic material at the center frequency of the operating range of the tube, for propagating therealong a wave having predominantly axial electric iield components; and means for projecting electrons along said structure in paths near the boundaries of said apertures in said loading disks for interaction with said iield components.

5. A traveling wave tube as in claim 4, wherein said loading disks and ceramic rings are sealed together to form part of the vacuum envelope of the tube.

6. A traveling wave tube as in claim 4, wherein alternate ones of said loading disks are conductively connected together for direct currents.

'7. The combination of the traveling wave tube as in claim 6 with direct current source means, connected to the two sets of alternate loading disks, for maintaining said sets at ditte-rent positive direct current potentials to focus said beam.

8. A traveling wave arnplitier tube as in claim 4, further including RF input coupling means coupled to the upstream end of said slow wave structure, and RF output coupling means coupled to the downstream end of said structure.

9. A traveling wave tube adapted to `operate over a relatively wide frequency range, including: a slow Wave structure comprising an elongated conductive rod, a series or" identical equally-spaced loading disks having central apertures coaxially surrounding said rod in spaced relation thereto, and a ser-ies of identical ceramic rings of high dielectric constant each interposed between and joined to the peripheral portions of two adjacent ones of said loading disks and having a radial thickness substantially equal to a quarter wavelength in the ceramic material at the center frequency of said range; and means for projecting a hollow beam .of electrons through said apertures in the annular space between said rod and said disks.

10. A traveling wave tube adapted toy operate over a relatively wide frequency range, including: a slow wave structure comprising an elongated conductive rod, a series of identical equally-spaced loading disks having central apertures coaxially surrounding said rod in spaced relation thereto, and a series of identical ceramic rings of high dielectric constant each interposed between and joined to the peripheral portions of two adjacent ones of said loading disks and having a radial thickness substantially equal to a quarter wavelength at the center frequency of said range, said loading disks having an annular yarray of sets of axially-aligned beam openings closely surrounding said central apertures; and means lfor producing and directing a separate electron beam through each set or aligned beam openings.

-l l. A traveling wave tube including: a slow wave structure, comprising an elongated conductive rod, a series of identical equally-spaced loading disks having aligned central apertures coaxially surrounding said rod in spaced relation thereto, and a series of identical ceramic rings of high dielectric constant each coaxially interposed between and joined to two adjacent ones of said loading disks atan annular region spaced from said apertures and having a radial thickness substantially equal to a quarter wavelength -in the ceramic material at the center frequency of the operating range of the tube, for propagating therealong =a Wave having predominantly axial electric iield components; and means for projecting electrons along said structure in paths near the boundaries of said apertures in said loading disks for interaction with said eld components, the dielectric constant of said ceramic rings being at least 25.

References Cited in the le of this patent UNITED STATES PATENTS 2,828,440 Dodds et al. p Mar. 25, 1958 2,843,793 Ashkin July 15, 1958 2,845,571 Kazan July 29, 1958 2,922,919 Bruck lan. 26, 1960 2,986,672 Vaccaro et al. May 3-0, 19611 FOREIGN PATENTS 902,160 Germany lan. 18, 1954 1,069,790 Germany Nov. 26, 1959 

1. A TRAVELING WAVE TUBE INCLUDING: A SLOW WAVE STRUCTURE COMPRISING AN ELONGATED CONDUCTOR, A SERIES OF IDENTICAL EQUALLY-SPACED LOADING PLATES HAVING ALIGNED CENTRAL APERTURES SURROUNDING SAID CONDUCTOR IN SPACED RELATION THERETO, AND LOW IMPEDANCE MEANS EFFECTIVELY CONNECTING PERIPHERAL REGIONS OF ADJACENT LOADING PLATES FOR RADIO FREQUENCY CURRENTS WHILE INSULATING SAID PLATE FROM EACH 